Problem: Suppose the diameter of a circle is ${10}$. What is its area?
Solution: ${d = 10}$ ${K = 25\pi}$ First, find the radius: $r = d/2 = {10}/2 = {5}$ Now find the area: $K = \pi r^2$, so $K = \pi \cdot {5}^2 = {25\pi}$.